In the field of obstetrics, one of the most important problems is the management of preterm labor and premature birth as they represent a major cause of perinatal morbidity and mortality.
For the treatment of preterm labor the use of magnesium sulfate and ethanol has been suggested. However, magnesium sulfate at plasma concentrations above the therapeutic range of 4 to 8 mg/dL can cause inhibition of cardiac conduction and neuromuscular transmission, respiratory depression and cardiac arrest, thus making this agent unsuitable notably when the renal function is impaired.
Ethanol is effective in preventing premature labor, but it does not produce a corresponding reduction in the incidence of fetal respiratory distress. Also, ethanol is assumed to have a negative impact on the fetus.
Two further therapeutical agents fall into either of the groups of:                a) β2-adrenergic agonists, or        b) oxytocin antagonists.        
The β2-adrenergic receptor generally causes an inhibitory action within the cells wherein it is expressed (muscles, heart, uterus etc). β2-adrenergic agonists are used to activate said inhibitory action of the receptor. Hence, β2-adrenergic agonists are sympathomimetics which—among others—inhibit uterine contractility. Known β2-adrenergic agonists for the treatment of preterm labor are Ritodrine, Terbutaline and Albuterol.
Ritodrine (i.e. (R*,S*)-4-Hydroxy-.alpha.-[1-[[2-(4-hydroxyphenyl)ethyl]amino]ethyl]benzenemethanol; see U.S. Pat. No. 3,410,944 of N. V. Philips) is the leading β2-adrenergic agonist but causes a number of cardiovascular and metabolic side effects in the mother, including tachycardia, increased renin secretion, hyperglycemia (and reactive hypoglycemia in the infant).
Terbutaline (i.e. 5-[2-[(1,1-Dimethylethyl)amino]-1-hydroxyethyl]-1,3-benzenediol, U.S. Pat. No. 3,937,838, Draco) and Albuterol (α1-[[(1,1-Dimethylethyl)amino]methyl]-4-hydroxy-1,3-benzenedimethanol; U.S. Pat. No. 3,644,353, Allen and Hanburys) are further β2-adrenergic agonists and have side effects similar to those of Ritodrine.
A more recent approach of treating preterm labor consists in the use of oxytocin antagonists.
Oxytocin (OT) is a peptide hormone and causes the contraction of the uterus of mammals during labor. The corresponding oxytocin receptor is similar to V1a and V2 vasopressin receptors and acts via a G protein-coupled receptor, coupled to activation of phospholipase C and increases in IP3 that release Ca2+ from intracellular stores. The increases in intracellular calcium that ensue lead to increased contraction of smooth muscle via activation of myosin light chain kinase. Oxytocin (OT) receptors increase dramatically during the course of pregnancy. The concentration of OT receptors has been shown to correlate with spontaneous uterine activity (M. Maggi et al. J. Clin. Endocrinol Metabol; 70; 1142, 1990). In the last few years, evidence has accumulated to strongly suggest that the hormone oxytocin may be a physiological initiator of labor in several mammalian species including humans. Furthermore, oxytocin is believed to exert this effect in two different parts:                by directly contracting the uterine myometrium and        by enhancing the synthesis and release of contractile prostaglandins from the uterine endometrium/decidua. These prostaglandins may, in addition, be important in the cervical ripening process.        
By these mechanisms, the process of labor (term and preterm) is initiated by a heightened sensitivity of the uterus to oxytocin, resulting in part as a result of an increase in the number of oxytocin receptors in this tissue.
By blocking oxytocin, the direct (contractile) and indirect (enhanced prostaglandin synthesis) effects of oxytocin on the uterus may be achieved. An oxytocin blocker, or antagonist, is therefore assumed to be more efficacious for treating preterm labor than current regimens.
Atosiban (i.e. oxytocin, 1-(3-mercaptopropanoic acid)-2-(O-ethyl-D-tyrosine)-4-L-threonine-8-L-ornithine) is a cyclic pentapeptide which is the best known OT antagonist (WO 9501368, Ferring A B; J. Reprod. Fertil., 101(2), 345–52 (English) 1994; Am. J. Obstet. Gynecol., 170(2), 474–8 (English) 1994). The principal drawback to the use of peptide antagonists like atosiban is the problem of low oral bioavailability resulting from intestinal degradation. Hence, they must be administered parenterally.
Also, WO 96/22775 and U.S. Pat. No. 5,756,497 (Merck) disclose benzoxazinyl-piperidines or benzoxazinones as OT receptor antagonists. Indanylpiperidines and tolyl-piperazines are reported by Evans et al. in J. Med. Chem., 35, 3919 (1992) as being orally deliverable OT antagonists
It has now been found that compounds of the present invention are antagonists of oxytocin and bind to the oxytocin receptor. When the oxytocin receptor is bound by the compounds of the present invention, oxytocin is antagonized by being blocked from its receptor and thus being unable to exert its biologic or pharmacologic effects. The compounds of the present invention are therefore useful in the treatment and prevention of preterm labor and premature birth. The compounds are also useful for stoppage of labor preparatory to cesarean delivery. In particular the compounds of the present invention are useful in the treatment and prevention of oxytocin-related disorders of animals, preferably mammals and especially humans. It is another purpose of this invention to provide a method of antagonizing the functions of oxytocin in disease states in mammals. It is also a purpose of this invention to develop a method of preventing or treating the oxytocin-related disorders of preterm labor by antagonizing the binding of oxytocin to its receptor.
The compounds of the present invention are also useful in the treatment of dysmenorrhea which may be defined as a cyclic pain associated with menses during ovulatory cycles. The pain is thought to result from uterine contractions and ischemia, probably mediated by the effect of prostaglandins produced in the secretory endometrium. By blocking both the direct and indirect effects of oxytocin on the uterus, an oxytocin antagonist is more efficacious for treating dysmenorrhea than current regimens.